In the field of vehicles operated by combustion engines, there is a continuing demand for low emissions of harmful substances in the exhaust gases from the vehicle's engine. These harmful substances are primarily in the form of pollutants, such as NO.sub.x compounds (NO, NO.sub.2, and N.sub.2 O), hydrocarbons (HC) and carbon monoxide (CO). In today's vehicles having gasoline engines, purification of the exhaust gases is normally carried out by means of an exhaust catalyst, which forms part of the exhaust system, and through which the exhaust gases are guided. In a so-called three-way catalyst, a major portion of the above-mentioned harmful compounds are eliminated through known catalytic reactions.
A condition for such a catalyst to operate with an optimal degree of purification is that the engine be controlled so that stoichiometry is obtained, i.e. so that a correctly adjusted air/fuel mixture is fed to the engine during its operation. In a motor vehicle, this can be accomplished by equipping the vehicle with a lambda sensor, by means of which a value of the oxygen content in the exhaust gases can be determined and fed to a control unit, which in turn generates the correct air/fuel mixture. However, if the condition regarding stoichiometry is not fulfilled, only a limited degree of purification of the catalyst can be obtained, particularly as regards the nitrogen oxides contained in the exhaust gases.
In certain types of combustion processes, for example in connection with diesel engines and in so-called "lean burn" processes, combustion is carried out with a relatively high surplus of oxygen, the purpose of which is to reduce the fuel consumption. This means that the stoichiometry condition is not fulfilled, and no purification of harmful emissions can be obtained in an effective manner with a conventional three-way catalyst. It is true that emissions of HC and CO compounds can be reduced by means of an oxidation catalyst, but the NO.sub.x emissions still constitute a problem. For this reason, there is a demand for devices and methods for the purification of NO.sub.x compounds, and particularly in combustion processes in which an oxygen surplus is present.
As regards the harmful NO.sub.x compounds, from the article "Removal of NO in the presence of 0.sub.2 using electrochemical cells," J. Nakatani, Second EU-Japan Workshop, Fundamental aspects of catalysis for clean combustion, Kyoto, Japan, Oct. 30-31 1995, a device is known by means of which NO.sub.x compounds can be converted to nitrogen gas. The device therein comprises a tube of an electrolytic material with a cathode which is arranged on the outside of the tube and an anode which is arranged on the inside of the tube. Helium is guided through the tube and past the anode, whereas a sample gas comprising nitrogen oxide, gaseous oxygen and helium is guided past the cathode. By applying an electric potential, nitrogen oxide can be converted into gaseous nitrogen and oxygen on the cathode, by means of which the ionized oxygen is pumped through the electrolyte to the anode.
The known device involves a disadvantage due to the fact that it cannot operate with the intended effect at high concentrations of oxygen in the gas in which the nitrogen oxides are to be reduced. This is due to the fact that it is not capable of distinguishing the NO.sub.x compounds from oxygen to any high degree. In the known device, oxygen must be pumped away before the conversion of NO.sub.x compounds begins. This known device thus provides a reduction of NO.sub.x compounds which is entirely insignificant when the oxygen concentration reaches approximately 2%. Consequently, since the device cannot operate at high oxygen concentrations, it cannot be used in an effective manner in connection with lean combustion processes, i.e. processes involving an oxygen surplus. An example of such a process is the combustion realized in diesel engines, the exhaust gases of which have an oxygen concentration which is approximately 5 to 18%.